1. Field of the Invention
The invention relates to scintillation crystal modules. More particularly, the invention relates to a scintillation crystal module having an improved seal (adhesive gasket cement) which maintains hermeticity after exposure to repeated thermal cycling.
2. State of the Art
Scintillation crystal modules or "gamma cameras" are widely used in nuclear medicine to non-invasively observe the interior of the human body. A gamma camera generally includes a single relatively large scintillation crystal, typically about sixteen inches by eighteen inches, and an array of photo-detectors optically coupled to the crystal. A source of gamma radiation (e.g. an isotope) is introduced into a patient's body and the gamma camera is placed against the patient's body. As gamma rays are emitted from the isotope, they pass through the body of the patient and strike different areas of the crystal causing it to release photons (i.e. emit light). The array of photo-detectors senses the relative brightness of light emitted from different parts of the crystal. Signals from the detector array may be processed in different ways to obtain either a two dimensional representation of the location of the isotope or an image of the region where the isotope is located.
Gamma cameras are relatively expensive, costing on the order of $50,000. Moreover, the scintillation crystal is extremely hygroscopic and it deteriorates quickly if exposed to moisture. Therefore, the crystal must be housed in a casing which will protect it from moisture, yet still permit gamma rays to excite the crystal and permit light from the crystal to be detected by an array of photo detectors.
A state of the art gamma camera is shown in prior art FIGS. 1 and 2. The camera 10 generally includes a scintillation crystal 12, typically thallium doped sodium iodide, an aluminum casing (or backcap) 14, a glass window 16, typically soda lime (K+ glass or PYREX), and an epoxy seal 18. The crystal 12 is adhered to the glass 14 by a layer of gel 20 having an index of refraction close to that of the glass. The epoxy seal 18 forms an hermetic seal between the glass 16 and the casing 14 to protect the crystal 12 from moisture. An array 22 of photo-detectors is coupled to the glass window 16. In use, electrical outputs (not shown) from the array 22 are coupled to an image processor (not shown). The aluminum casing 14 is relatively thin (approximately 0.040") so that it has a low gamma ray absorption.
A problem common to all known gamma cameras is that the epoxy seal 18 is significantly weakened if the camera 10 is exposed to extreme temperature cycles. While such exposure and consequent weakening of the seal does not normally result in immediate hydration of the crystal, it does significantly shorten the useful life of the crystal. Most gamma cameras in use today have a useful life of one to two years. As the camera ages and the seal is weakened, the crystal becomes cloudy as it is hydrated, and its performance is reduced until it is unsuitable for imaging purposes.
It is generally understood in the art that the coefficient of thermal expansion (CTE) of the aluminum casing is very different from the CTE of the glass window. The CTE of the K+ glass is approximately 5.times.10.sup.-6 in/in.degree. F. and the CTE of the aluminum casing is approximately 13.3.times.10.sup.-6 in/in.degree. F. It is also understood that this difference in the CTEs causes the glass and the aluminum to move relative to each other when the ambient temperature changes. It is generally believed that this movement induces stress on the epoxy seal and causes it to separate from the glass and/or the aluminum. If the ambient temperature change is sufficient (e.g. 40.degree. F.), the bond between the epoxy and the glass or the aluminum indeed can be broken. A 40.degree. F. change in ambient temperature is not uncommon when a gamma camera is being shipped to a location for use. Thus, many gamma cameras may actually be damaged before their initial use. As mentioned above, damage to the epoxy seal does not usually result in catastrophic failure of the camera crystal. However, as mentioned before, the crystal often begins a slow process of hydration after the initial damage to the seal.
The general understanding of the effects of thermal expansion on gamma cameras is illustrated in FIG. 3. Since the CTE of the backcap 14 is approximately 2.7 times that of the glass 16, for a given change in temperature, the backcap 14 will expand or contract approximately 2.7 times as much as the glass 16. FIG. 3 shows the relative positions of backcap 14 and the glass 16 at 70.degree. F. The phantom lines 14a, 16a show the relative positions of the backcap 14 and the glass 16 at 40.degree. F. The general understanding in the art, therefore, is that when the camera is cooled, the seal 18 is subjected to compression forces by the contracted backcap 14. Similarly, it is generally believed that when the camera is heated, the seal 18 is subjected to tensile forces as the space between the glass and the backcap increases.
Recognizing the effects of thermal expansion, several solutions have been proposed by those skilled in the art to reduce the stress exerted on the seal. Traditionally, the approach to increase seal performance was focused in two areas. The first approach involved the use of an epoxy with a higher tensile strength or lower elastic modulus (softer). The second approach attempted to reduce the difference, or mismatch, between the CTE of the backcap and the CTE of the glass window. It was believed that if the CTE of the glass and the CTE of the backcap could be closely matched, the stresses in the seal during temperature cycling could be drastically reduced. This second approach was never successfully implemented due to the limited availability of suitable materials and the difficulty of manufacturing a camera from materials other than aluminum. The current aluminum backcaps provide low gamma ray absorption by using 0.040" thick aluminum. Aluminum of this thickness is rigid enough to avoid damage during handling, and is easy to manufacture with hydroforming techniques. While a steel backcap would have a CTE more closely matched to glass, the thickness of a steel backcap would have to be reduced to 0.005" in order to maintain an acceptable level of gamma ray attenuation. Such thin steel would not be as durable as the thicker aluminum and would not be easy to manufacture.
More recently, efforts have been made to engineer the geometry of the seal and the aluminum casing to reduce the stress imparted on the seal. U.S. Pat. No. 5,148,029 to Persyk et al. discloses a gamma camera having an inner seal and an outer seal. The Persyk et al. disclosure is based on the assumption that the separation of the seal from the glass and/or the casing is "because the aluminum casing and the glass window have significantly different thermal expansion coefficients and the forces which are generated during temperature changes cause the seal to break free of the parts to which it is attached."
U.S. Pat. No. 5,229,613 to Pandelisev et al. discloses a gamma camera constructed from a stainless steel ring within which an x-ray window, a scintillation crystal, and a PYREX optical window are arranged. The two windows are sealed to the steel ring by laser welding or soldering using an Indium-Tin alloy or other suitable material. An annular cut is provided in the steel ring surrounding the optical window "to provide thermal relief to the outer ring and the transparent plate" and it is filled with a rubber filler. Rubber filler is also provided between the x-ray window and the steel ring. This complex camera avoids the use of an epoxy seal, but is still based on the assumption that the comparative thermal properties of the glass and the casing are the cause of seal separation.